PRINCIPLES OF GENETICS 



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are limits to the subdivision of the germinal material. 

 For example in the vinegar fly, Drosophila melanogaster, 

 there are known about 400 new mutant types that fall 

 into only four linkage groups. 



Fig. 9. 

 Cross between a sweet pea with purple flowers and long pollen 

 grains and one with white flowers and round pollen grains. In the 

 lower line the four classes of Fo individuals appeared in the pro- 

 portions given. 



One of these groups of characters of Drosophila is said 

 to be sex-linked, because in inheritance the characters 

 show certain relations to sex. There are about 150 of these 

 sex-linked mutant characters. Several of them are modifi- 

 cations of the color of the eye, others relate to its shape 



